Pipets are used to extract, measure and transfer a volume of liquid. A common pipet design is a cylindrical vessel open at both ends, like a drinking straw, with a mark to indicate a predefined volume and a means to apply suction and pressure to one end of the cylinder. In use, suction is applied to draw liquid into the pipet from a reservoir. The pipet is then moved to a receiving vessel and pressure or gravity empties measured volume.
An important concept in volumetric measurement of liquid is that precision can be increased by reducing the diameter of the cylinder at the upper limit of the liquid. This is seen in volumetric flasks, which have a large diameter base and a smaller diameter neck at the top. Volumetric pipets are usually cylindrical, as previously stated. However, volumetric pipets may have a large diameter section, e.g., in the middle, so that the pipet can hold more volume, although the area of the pipet corresponding to the limiting point for the liquid is typically narrowed again. Stated another way, reducing the surface area of the meniscus increases the accuracy.
Most pipets fill from the bottom end, while pressure and suction are applied at the upper end. This configuration has the advantage of minimizing the surface area of the boundary between the liquid and the air, thereby maintaining precision. Another advantage of this simple yet effective device is that the liquid to be measured does not contact any valves. The liquid is suspended in the pipet with suction. A disadvantage associated with the use of valves is that valves have mating surfaces, seats, and fittings having irregular surfaces that are likely to retain the liquid, reduce precision and make the device more difficult to rinse.
One disadvantage of the bottom fill pipet described above is that the pipet must be moved with each cycle of operation from the fill location to the dispense location. Another disadvantage is that the pressure and suction must be carefully controlled.
In addition to manual pipets, automated pipet systems have been developed. An example automated pipet system may include a syringe, a stepper motor, a three-way valve to select between intake and dispense functions, and equipment necessary to move the pipet vertically in and out of a fluid as well as equipment necessary to move the pipe horizontally from an intake location to a dispense location. Although a means to apply pressure and suction has been automated and the movement of the pipet in the x and y directions has been automated, typically the same basic design is used, wherein a cylindrical vessel is opened at both ends. Examples of typical “glass straw” pipet vessels may be found in U.S. Pat. Nos. 3,992,947, 4,476,095, 4,624,147, 5,090,255, 5,271,902, 5,679,575, 5,820,824, and 6,253,628.
Other known pipette designs include the unitary filter-pipette taught in U.S. Pat. No. 3,415,380 to Ellis. The Ellis pipette fills from the top and has the advantage of a measuring chamber that, by its design, holds a limited amount of liquid, making the volumetric measurement automatic. Ellis teaches a manual pipet.
One drawback associated with Ellis is that liquid may continue to enter the measuring chamber from the filter and its funnel while the pipet is emptying, which will compromise the accuracy. Additionally, liquid is supported by a valve at the bottom of the measuring chamber, which will reduce precision and complicate rinsing. Rinsing requires either moving the device or replacing the receiving vessel after rinsing because the rinse media exits the device through the same port as measured liquid.
The above described accuracy limitations make the Ellis device inappropriate for high precision applications. Having to position the device for rinsing makes the Ellis device less suitable for automation.
U.S. Pat. No. 2,434,723 to Shook describes a Means for Measuring Volumetric Samples in that has the feature of isolating measure liquid between two valves. Shook teaches a manual rather than automated pipet.
Shook's device does not provide a clear or separate path for displaced air to evacuate when filled from the top. Rinsing is required between the fill and dispense operations. Otherwise, liquid will continue to enter the measuring chamber from the vessel above the uppermost valve while the measured liquid is emptying. Rinsing requires either moving the device or replacing the receiving vessel after rinsing, or turning a valve to select a separate passage for the rinse media. This device has a valve below and another above the measuring chamber. These valves will reduce precision and complicated rinsing.
German Patent No. 929,333 to Altmann describes a buret. The Altmann device fills from the top, using pressure or suction to fill from a supply reservoir that is at a lower elevation than the full mark of the buret. Altmann teaches that the buret is filled with excess fluid, then gravity and a siphon effect return the excess to a supply reservoir.
The Altmann device has a valve at the lower boundary of the measuring chamber, which will reduce precision and complicate rinsing. The use of pressure or suction requires that a source of pressure or suction be available. The Altmann device delivers the rinse media and the measured liquid through the same opening requiring that the rinse vessel and the sample vessel be moved back and forth. Only one opening is provided in the top of the measuring chamber through which air must evacuate and liquid enter, which limits accuracy. The Altmann buret is shown having a straight cylinder wall. Altmann shows the upper limit of the liquid to be at point D, the delivery tip. Further, Altmann's device requires that air must evacuate the measuring area of the buret and escape through opening H.
U.S. Pat. No. 4,476,095 to Scott describes an automated pipet that includes a motor driven syringe to supply suction, pressure and volumetric measurement, and a device to position the pipet in two locations, i.e., one location for filling the pipet and a second location for delivering the measured liquid. Scott teaches a “drinking straw” style pipet that has been automated and has many limitations. The Scott device is difficult to rinse and rinsing may require disassembly. It is complex because it has a motor driven syringe supplying pressure and suction, and a positioning device. Position devices and motor driven syringes require control circuits, motors, gears and maintenance.
Moon describes a Flow-thru-Pipet in U.S. patent application Ser. No. 10/208,420. The Moon device includes a fill chamber where sample preparation can occur. It uses compressed air to empty the measuring chamber. The Moon pipet fills from the top. The measuring chamber has separate passages to simultaneously fill with liquid and evacuate air. To rinse the device, the receiving vessel must be replaced because the rinse media and measured liquid are dispensed through the same port. The device also has a valve at the lower end of the measuring chamber, which compromises precision and complicates rinsing.
A drawback with bottom fill manual and automated pipets is that fluid is drawn into the pipet and dispensed from the pipet through the same orifice, which is usually located at the lower end of the pipet. Filling and dispensing of fluid from the same orifice in the pipet necessitates locating the pipet in a fluid source to fill the pipet and then relocating the pipet at a dispensing location every time it is desired to dispense a sample of fluid. Consequently, automated pipet systems require complex systems to relocate the pipet from the fluid source to the dispensing location.
Therefore, a pipet is desirable that is capable of delivering a repeatable predetermined volume of fluid, wherein the pipet fills from the top and dispenses from the bottom, i.e., a “flow through” pipet, which would eliminate the need for positioning devices. It is further desirable to provide a top fill pipet that does not trap air in the measuring chamber. Such a pipet could be provided in an automated pipetting system wherein the pipet would not have to be repositioned to a fill location after dispensing a fluid sample, thereby greatly simplifying an automated pipetting system.
It is further desirable to have a system that automates titrations that use volumetric measurement of the sample. There are many titration methods that are specific to a sample and chemical species being measured. Generally a titration method is comprised of four steps: 1) Sample preparation, which may be a chemical addition or physical manipulation of the sample; 2) Sample measurement by weight or volume; 3) Titration, which is the addition of a chemical of known concentration until a desired reaction occurs; and 4) Calculation of the concentration of the sample. Sample preparation may include more than one step and may occur before or after the sample measurement. To summarize, it is desirable to have a system that automates all four steps above and does not require any human input during the sample preparation step through the calculation steps described above.
Therefore, it is desirable to accomplish sample measurement by volume, and to provide a vessel where sample preparation can occur when necessary and prior to sample measurement. Until now, volumetric measurement has been manually accomplished with volumetric pipets, volumetric flasks, and in a limited number of situations with automated pipets.
It is additionally desirable to provide a pipet that includes a vessel or chamber for sample preparation wherein the pipet is capable of dispensing a measured volume that meets the precision requirements of a class A volumetric pipet as specified in ASTM E969-02, which is plus or minus 0.08 milliliters for a 100 ml pipet. One drop of liquid is usually about 0.05 ml. Therefore, the device is accurate to approximately a single drop of fluid.
It is further desirous to eliminate the need for any positioning devices including both devices for moving the pipet and devices for moving the sample vessel or rinse media vessel. Positioning devices are expensive and complex. Therefore it is beneficial to fill the pipet from the top so that gravity can be used to move the liquid to different locations.
It is desirous to eliminate the need for suction to fill the pipet, eliminating the need for an expensive vacuum supply or a suction pump. Therefore, it is beneficial to fill the pipet from the top allowing gravity to fill the pipet. Additionally, it is desirable that the device be configured for effective rinsing, thereby eliminating contamination of one sample by the previous sample. Also, rinsing should be simple and quick for minimizing operator time and skill. The device should be inexpensive to build, maintain and operate.
It is desirous to eliminate valves that contact the sample liquid in the measuring chamber. The “straw” style pipet does not have any valves in contact with the measured liquid. It is desirable to emulate this feature because it will benefit precision.